The present invention relates to a superconducting magnet for MRI and more particularly to a cooling structure therefor.
A superconducting magnet for MRI (Magnetic Resonance Imaging) is designed to produce a uniform magnetic field in a space defined between upper and lower doughnut-shaped, superconducting coils. In a hospital a patient is laid between the upper and lower superconducting coils to examine his or her internal organs and the like. Conventionally, a horizontal cylindrical superconducting coil has been used as a superconducting MRI magnet but there is a problem that a patient feels being cooped up. An MRI system using two superconducting coils, situated at upper and lower positions, is called an “open type” because a space opening in a horizontal direction is formed by the upper and lower superconducting coils. This “open type” MRI system is well received by doctors and patients since it gives little intimidatory feeling and oppressive feeling. This invention concerns the superconducting magnets for this “open type” MRI system.
As concerns a cooling construction for a superconducting magnet, JP-A-2001-244109 discloses a structure which is connected to a refrigerator through a heat transporting medium and removes heat from coils by operating the refrigerator. JP-A-11-176629 discloses a superconducting magnet device in which frames for superconducting coils are made of an aluminum alloy and a refrigerator is installed on the frames through a heat conducting member.
Further, JP-A-2002-270913 discloses a technique that uses a sensible heat of evaporated helium gas to cool a thermal shield in order to reduce a consumption of liquid helium while a refrigerator is stopped.
The superconducting magnets for MRI are used in hospitals mainly for clinical purposes. It is therefore required that the superconducting magnet should be quenched for a predetermined period of time even when the refrigerator is stopped. Conventional superconducting MRI magnets have a problem that, while the refrigerator is stopped, the superconducting magnet is quenched, and an MRI imaging device fails to perform its function and becomes unavailable.